An absorbent body, or absorbent pad, for use with absorbent articles, such as disposable diapers, sanitary napkins and incontinence guards, is normally comprised of one or more layers of cellulose fluff, and will more often than not include so-called superabsorbents, which are polymers capable of absorbing several times their own weight of water or body liquid. The absorbent body may also include other constituents, for instance constituents which will improve the liquid-spreading properties of the absorbent body, or constituents which will enhance the ability the absorbent body to hold together and to resist deformation during use.
One serious problem with present-day absorbent articles, and primarily in the case of diapers and incontinence guards which are intended to receive and absorb relatively large quantities of liquid, is that the articles often leak long before their total absorption capacity has been fully utilized. One reason for this leakage is that those absorbent bodies known hitherto are unable to rapidly take-up and absorb large liquid quantities effectively.
Although an absorbent body which includes superabsorbents has a high absorption capacity and is able to retain the liquid absorbed, even when the absorbent body is subjected to external pressure, the superabsorbents have an extremely low absorption rate. Since urination often involves the discharge of large quantities of liquid over a period of some few seconds, the absorbent body will often become saturated with liquid temporarily, so that any further liquid that is discharged will flow out of the absorbent body.
This premature leakage is naturally very irritating to the person concerned, or to the person nursing the afflicted person. The fact that they may not be able to rely on the proof of an incontinence guard against leakage is a constant worry to adult incontinents who are otherwise healthy people and who depend on an effective guard in order to be able to enjoy a normal working and social life. Furthermore, premature leakage of children's diapers and adult diapers and incontinence guards means that wet clothes and bed linens must be changed and washed more often than would otherwise be necessary.
Great efforts have been made in the past to produce an absorbent body which is capable of rapidly absorbing large quantities of liquid. For example, U.S. Pat. No. 3,441,023, U.S. Pat. No. 4,276,338, EP 124 365, U.S. Pat. No. 4,333,462, EP 343 940 and EP 343 941 propose that cavities or basin-like hollows are formed in the absorbent body.
Serious endeavours have also been made in attempts to control the flow of liquid in the absorbent body, by compressing different layers or regions of said body to different high degrees or in different patterns. Variations in the degrees of compression also varies the capillary structure of the absorbent body and therewith also its liquid transporting ability. Since liquid is always transported from the coarser to the finer capillaries, liquid will be transported in such structure from the regions of low degrees of compression to regions of high degrees of compression. It is difficult, however, to create density differences which are sustained both in a wet and a dry state, simply by compression, since cellulose fluff tends to assume its critical bulk when wet. Critical bulk is the bulk at which a given cellulose fluff body will neither collapse nor expand when wetted. This means that a cellulose fluff body which has been compressed to a bulk beneath its critical bulk will swell when wetted, whereas a body whose bulk is higher than the critical bulk will collapse when wetted.
SE 458 418 describes an absorbent body which has a density gradient that has been made wet-stable with the aid of a heat meltable binder. One drawback with an absorbent body of this kind is that the binder impairs the absorbency of the body. Another drawback is that the binder is relatively expensive and therewith increases the cost of the finished absorbent body, which is particularly unsuitable in the case of absorbent bodies which are intended for one-time-use only.
In order to increase the ability of an absorbent body to absorb a large quantity of liquid over a short period of time, it is proposed in EP 0 325 416 that a low density region is provided in that part of an absorbent body which is intended to face forwards on a wearer in use. The intention with this particular arrangement is that discharged body liquid can be quickly taken-up in the relatively coarse capillaries in the region of low density and then be absorbed by the surrounding material of high density. Because of the differences in the size of the capillaries between the liquid-receiving region and the surrounding material the liquid-receiving region will be emptied of liquid to a significant extent before liquid is next delivered to the absorbent body.
One serious drawback with an absorbent body according to EP 0 325 416, however, is that it is difficult to maintain the low density in the liquid-receiving region when the body is in use, particularly when the absorbent body is comprised of cellulose fluff. When the absorbent body is used in a diaper for instance, the liquid-receiving region is located in the user's crotch and there subjected to strong mechanical influences by the movements of the wearer's legs or thighs. Consequently, the absorbent pad becomes deformed and is compressed together or pleated into string or sausage-like shape. The liquid-receiving region therewith loses its shape and can be more or less rendered totally ineffective during use. Such deformation is particularly noticeable in a wet absorbent body which is made of cellulose fluff. Such an absorbent body can therefore only be wetted once, before the risk of leakage becomes serious.
Another problem with absorbent articles is one of keeping the surface that faces towards the wearer in use as dry as possible during the whole period of use and to prevent so-called re-wetting. Re-wetting occurs when absorbed liquid is pressed out of the absorbent body and wets the skin of the wearer. This is prevented to some extent when the absorbent body contains superabsorbents, which chemically bind the absorbed liquid even when the article is subjected to external pressures, for instance when the wearer sits down. One difficulty, however, is that of constructing the absorbent body in a manner such that the liquid will spread within the body and reach the superabsorbent material.
U.S. Pat. No. 4,047,531 describes an absorbent article whose absorbent body includes a first layer which is comprised of mechanical fluff thermomechanical fluff or semichemical fluff, and a second layer which is comprised of a material different to that of the first layer and consists of thermomechanical fluff, semichemical fluff or chemical fluff. The materials in the two layers are chosen so that the fibres in the first layer are more hydrophobic than the fibres in the second layer.
In a first embodiment, the first layer is comprised of thermomechanical fluff, while the second layer is comprised of chemical fluff. By using thermomechanical fluff in the first layer, there is obtained a higher yield of the wood raw material and it is said that such fluff is more environmentally friendly in manufacture than chemical fluff. It is also said that less energy is required to defibre sheets of thermomechanical fluff. The risk of re-wetting is also reduced with the use of two different fluffs.
Chemical fluff will collapse when wet and therewith obtain a dense structure of good liquid-spreading ability but of low liquid retention. An absorbent body which is comprised solely of chemical pulp will result in a high degree of re-wetting when subjected to pressure. On the other hand, thermomechanical fluff is highly resilient or springy when wet, and therewith is more able to resist compression. Because the materials in the two layers have different degrees of hydrophobicity, absorbed body liquid is transported from the more hydrophobic layer to the layer which is less hydrophobic. It is said that the risk of re-wetting is minimized in this way.
U.S. Pat. No. 4,047,531 uses the terms "mechanical", "thermomechanical", "semimechanical" and "chemical" to characterize different types of cellulose fluff. These terms, however, are not sufficiently defined to disclose clearly those properties that are required of the fluffs in order for the resultant absorbent body to obtain the properties stated. The terms used merely indicate the properties that can be expected of the fluffs with a starting point from the stated methods of manufacture. It is possible, however, to subsequently treat the fluffs in different ways and thereby change their hydrophobicities for instance, such as to completely reverse the order of hydrophobicity disclosed in U.S. Pat. No. 4,047,531.
Furthermore, the degree of hydrophobicity is only one of several parameters which control the transport of liquid in a fibre structure. Equally as important parameters include, for instance, the porosity of the structure and the relationship between the sizes of the capillaries in the body layers. A dense, relatively hydrophobic structure may thus have greater ability to spread liquid than a porous, less hydrophobic structure.
In order to be able to receive large quantities of liquid over a short period of time, an absorbent body will preferably have an open fibre structure which will accommodate a large quantity of liquid between its fibres. In this case, the hydrophobicity of the fibre structure is of less importance, provided that the liquid is able to penetrate into the absorbent body.